1. Field of the Invention
The present invention relates to an image reading method and apparatus for reading image signals in accordance with image information from a solid-state detector, where the image information is recorded by exposure to an electromagnetic wave for recording.
2. Description of the Related Art
Currently, in radiography for the purpose of medical diagnosis or the like, there is known a radiation image recording and reading apparatus using a radiation solid-state detector for detecting radiation and outputting image signals which represent radiation image information. As the detector used in such an apparatus, various types of detectors have been proposed and put into practice.
For example, in terms of a charge generation process of converting radiation into charges, there are known radiation solid-state detectors of an optical conversion type (for example, U.S. Pat. No. 4,803,359, Japanese Unexamined Patent Publication No. 2(1990)-164067, PCT International Publication No. WO92/06501, and SPIE Vol. 1443 Medical Imaging V; Image Physics (1991), p. 108-119, etc.) and radiation solid-state detectors of a direct conversion type (MATERIAL PARAMETERS IN THICK HYDROGENATED AMORPHOUS SILICON RADIATION DETECTORS, Lawrence Berkeley Laboratory, University of California, Berkeley, Calif. 94720, Xerox PARC, Palo Alto, Calif. 94304, Metal/Amorphous Silicon Multilayer Radiation Detectors, IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 36, NO. 2, APRIL 1989, Japanese Unexamined Patent Publication No. 1(1989)-216290, etc.). In the optical conversion type detectors, fluorescence emitted from a phosphor by exposure to radiation is detected by a photoelectric conversion device. Signal charges thereby obtained are once accumulated in an electric accumulator of the photoelectric conversion device, the accumulated charges are converted into an image signal (electric signal) and the image signal is outputted. In the direct conversion type detector, signal charges produced in a radiation conductive material by exposure to radiation are collected by a charge collection electrode and once accumulated in the electric accumulator. The accumulated charges are then converted into an electric signal and the electric signal is outputted.
In terms of a charge reading process of reading out the accumulated charges, there are known radiation solid-state detectors of a TFT (thin film transistor) reading type which read out the charges by scanning and driving TFTs connected to the electric accumulators, radiation solid-state detectors of an optical reading type which read out the charges by irradiating reading light (reading electromagnetic wave) on the radiation detectors, and the like.
There has also been proposed radiation solid-state detectors of an improved direct conversion type in U.S. Pat. Nos. 6,268,614 and 6,376,857. The radiation solid-state detectors of the improved direct conversion type employ a combination of the direct conversion type and the optical reading type. The radiation solid-state detector thereof includes a first electrode layer transparent with respect to recording radiation; a recording photoconductive layer which exhibits photoconductivity when exposed to the recording radiation transmitted through the first electrode layer; a charge transport layer which acts substantially as a insulator for charges of the same polarity as that of the charges accumulated in the first electrode layer and acts substantially as a conductor for charges of the opposite polarity to the same; a treading photoconductive layer which exhibits photoconductivity when exposed to the reading electromagnetic wave; and a second electrode layer transparent with respect to the reading electromagnetic wave, which are laminated in this order. Latent image charges bearing the image information are accumulated at the interface of the recording photoconductive layer and the charge transport layer.
As the method of reading the latent image charges in the radiation solid-state detector of the improved direct conversion type, the following methods are known. In one method, the second electrode is a flat plate, and the latent image charges are detected by scanning the second electrode layer with a spot-like reading light such as a laser beam. In another method, the second electrode is composed of a comb-teeth shaped stripe electrode, and the latent image charges are detected by scanning the second electrode layer with a linear light source in a longitudinal direction of the stripe electrode, the linear light source extending in a direction approximately perpendicular to the longitudinal direction of the stripe electrode.
In the optical reading method, the signal read from the flat plate electrode or the stripe electrode is obtained as pixel signals by sampling or the like. A region corresponding to the pixel signal represents a pixel in the radiation solid-state detector. In the TFT reading method, since the signal outputted from each transistor is read as the pixel signal, each transistor represents a pixel in the radiation solid-state detector.
A reproduced image is constituted based on an image signal composed of the pixel signals obtained as described above and provided for image diagnosis by display on a monitor or the like.
Here, the radiation solid-state detector as described above can repeatedly record the radiation image information and read the image signal in accordance with the radiation image information.
However, in the radiation solid-state detector as described above, the following problem occurs. If the radiation solid-state detector is exposed to radiation of excessive energy in recording of the radiation image, the sensitivity is lowered in a portion exposed to the radiation of excessive energy in recording and reading of the next radiation image. This is because the latent image charges produced by exposure to the radiation of excessive energy are not entirely read off and remains in the recording photoconductive layer as residual charges. The residual charges decrease the intensity of an electric field formed between the first electrode layer and the second electrode layer and decrease a charge conversion efficiency in the recording photoconductive layer. In the reading, the residual charges may also decrease a reading efficiency in the reading photoconductive layer. The decrease in the charge conversion efficiency and the decrease in the reading efficiency cause degradation of the S/N ratio of the reproduced image. The number of residual charges decreases over time, but a long period of time is required until the effect thereof is decreased to a negligible degree.
Not only in the radiation solid-state detector where the radiation image is recorded by converting radiation into charges, but also in the radiation solid-state detector where fluorescence is emitted by exposure to radiation and the radiation image is recorded by detecting the fluorescence, an efficiency of converting radiation into fluorescence is decreased by exposure to the excessive radiation, and the degradation of the image quality such as the S/N ratio of the image is caused similarly to the above radiation solid-state detector.